ABSTRACT Central leptin resistance results in decreased downstream signaling to brain circuits that regulate food intake and energy expenditure, thereby promoting hyperphagia and obesity. While the mechanisms leading to this leptin resistance have yet to be fully elucidated, there can be no doubt that the consequent failure of downstream circuits that regulate food intake are impaired, leading to increased adiposity. A circuit of oxytocin neurons in the hypothalamic parvocellular paraventricular nucleus (pPVN) that project to the nucleus of the solitary tract (NTS) in the hindbrain has recently emerged as a highly promising pathway that contributes to the ability of leptin to inhibit food intake. The research proposed in this application focuses on establishing key components of the mechanism by which oxytocin's ability to enhance the behavioral effects and hindbrain neuronal activation produced by meal-related satiety signals, such as cholecystokinin (CCK), are altered in states of diet-induced obesity (DIO). Defective activation of CCK-sensing neurons in the NTS and an impaired satiety response to CCK are predicted to result in enhanced susceptibility to DIO. Thus, these circuits are potential targets for pharmacological therapies to counter hypothalamic leptin resistance and reverse the deleterious metabolic effects of central leptin resistance. The studies outlined in Specific Aim 1 will establish the relationship between leptin-induced activation of oxytocin neurons in the hypothalamic pPVN and activation of neurons in the hindbrain NTS in response to the satiety signal CCK in a rodent model of DIO. Oxytocin mRNA expression will be measured in a rodent model of DIO by using laser capture microdissection (LCM) and RT-PCR. These techniques will be used to determine the impact of DIO on the effect of leptin to block fasting elicited decreases in oxytocin mRNA expression in the pPVN. Immunocytochemistry (ICC) and pharmacological approaches will be used to determine the effect of leptin to activate pPVN oxytocin neurons in DIO and whether this prevents leptin from enhancing cholecystokinin (CCK-8)-elicited neuronal activation in the NTS in DIO. The information gained from these studies will provide an anatomical basis for understanding the link between impaired activation of oxytocin in animal models of DIO and leptin resistance. The studies outlined in Specific Aim 2 will establish the relationship between reduced oxytocin signaling in the NTS and the behavioral and metabolic defects that accompany a rodent model of DIO. Oxytocin signaling will be reduced in the NTS by administering a lentivirus that expresses the shRNA to the rat oxytocin receptor, into the NTS of animals fed a low fat diet, and food intake, meal patterns, energy expenditure, locomotor activity, body weight, and responsiveness to leptin will be examined relative to DIO rats. This compound was recently found to decrease oxytocin receptor protein expression in human myometrial cells that express oxytocin receptors. These findings will provide crucial evidence demonstrating an important causal role for reducing oxytocin signaling in the NTS and a predisposition to the behavioral and metabolic effects that accompany DIO. The studies outlined in Specific Aim 3 will establish the relationship between increased oxytocin signaling and leptin resistance in a rodent model of DIO. To accomplish this, the effects of chronic systemic administration of oxytocin on food intake, meal patterns, energy expenditure, locomotor activity, and body weight will be examined. Furthermore, we will measure the effects of chronic systemic administration of oxytocin alone and in combination with CCK-8 as a strategy to prevent DIO. Together, these findings are crucial to understanding the potential efficacy of interventions in the PVN-NTS oxytocin signaling circuit to the pathogenesis underlying DIO, and thereby direct future research on oxytocin signaling as a potential therapeutic target for preventing or ameliorating DIO.